The present invention relates to production of semiconductor devices and, in particular, to formation of a shallow junction due to shallow doping of impurity into a surface layer of a semiconductor substrate.
As semiconductor devices have recently been densified and small-sized, it has been demanded in MOS transistors that a diffused junction is formed at a shallow depth from a surface of source or drain region in the substrate. For example, the depth of the diffused junction is actually known about 0.1 .mu.m in 64M (Megabits) DRAM (Dynamic Random Access Memory) and 0.08 .mu.m in 256M DRAM. It is estimated to be 0.06 .mu.m in 1G (Gigabit) DRAM.
In order to form such a shallow junction, there are two problems to be resolved, one of which is shallow doping of impurity, and the other being a low heat load for a heat-treatment or an annealing of the substrate doped with the impurity. The latter, that is, the low heat load is necessary for the doped impurity to be prevented from diffusing into the substrate during the heat treatment. This is realized by performing the heat treatment at a low temperature or performing the heat treatment for a short time at a high temperature such as a method of, so called, lamp annealing. Techniques to resolve the latter problem have been studied and effective ones have been developed.
However, the former problem is still remained as an important problem.
The ion implantation is mainly used for doping impurity into the semiconductor substrate. In order to form the shallow junction by use of the ion implantation, ion implantation must be performed at a low voltage such as several kilovolts. This is important for ions of boron which is used as p-type impurity because the lighter mass boron penetrates a deeper region. However, it is difficult in use of the low voltage to maintain a high electric current stable.
A plasma doping method has been proposed for implanting impurity ions into the substrate at a low voltage. According to the method, B.sub.2 H.sub.6 or PH.sub.3 gas is mixed with H.sub.2 or He gas to form a mixed gas, and a DC voltage of about 900 V is applied between electrodes in the mixed gas to form a plasma so that ions of B or P are formed and accelerated by the electric field and are implanted into the substrate disposed in the mixed gas. Although the impurity doped region can be limited to a shallow region of the substrate, a problem is that activation of the doped impurity is not sufficient.
In order to resolve the problem, Japanese patent application laid open with a number of JP-A-1 120015 proposed another ion implantation method where the impurity gas was diluted by Ar gas of a high purity. In detail, the impurity gas such as B.sub.2 H.sub.6 gas is introduced together with Ar gas in a plasma doping apparatus under regulation of its flow rate, and the plasma is generated by application of DC voltage such as about 1,000 volts or less to perform doping B ions. As a result, a boron doped region is formed with a carrier concentration of 1.times.10.sup.21 cm.sup.-3 which is excessive of solid solubility.
According to the description of JP-A-1 120015, the low activation of the impurity doped by the plasma doping method is caused by a fact of use of hydride or fluoride of the impurity because the impurity is connected with hydrogen or fluorine to form connections such as B--H, P--H, Ac--H or B--F in the doped region. In order to prevent connection of the impurity with hydrogen or fluorine, Ar gas is used together with the hydride or fluoride gas of the impurity so that Ar ions are doped into the substrate together with the impurity ions. Thus, high activation of the doped impurity is insured.
In the method proposed in JP-A-1 120015, a large amount of Ar ions are doped into the substrate and therefore, generate voids in the Ar doped region by the heat treatment after doping. Since boron ions and phosphorus ions are doped deeper than Ar ions, the diffused junction is not damaged by the doped Ar ions and free from degradation of property. However, impurity ions such as As ions are doped shallower than Ar ions and the diffused junction is therefore degraded by the Ar ion doping.